Introduce a generic interface for querying an operation's expected
lowered cost.

Currently, this is a direct port of the logic implementing
isInstructionFree in CodeMetrics. The hope is that the interface can be
improved (f.ex. supporting un-formed instruction queries) and the
implementation abstracted so that as we have test cases and target
knowledge we can expose increasingly accurate heuristics to clients.

I'll start switching existing consumers over and kill off the routine in
CodeMetrics in subsequent commits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172998 91177308-0d34-0410-b5e6-96231b3b80d8